The present invention relates to a surface shape recognition sensor used to sense a surface shape having a fine three-dimensional pattern such as a human fingerprint or animal noseprint.
Along with the progress in information-oriented society in the environment of the current society, the security technology has received a great deal of attention. For example, in the information-oriented society, a personal authentication technology for establishment of, e.g., an electronic cash system is an important key. Authentication technologies for preventing theft or illicit use of credit cards have also been extensively researched and developed (e.g., Yoshimasa Shimizu et al., “A Study on the Structure of a Smart Card with the Function to Verify the Holder”, Technical Report of IEICE OFS92-32, pp. 25-30 (1992-11)).
There are various kinds of authentication schemes such as fingerprint authentication and voice authentication. Especially, many fingerprint authentication techniques have been developed so far. Fingerprint authentication schemes are roughly classified into an optical reading scheme and a scheme of using the human electric characteristic and detecting the three-dimensional pattern of the skin surface of a finger and replacing it with an electrical signal.
In the optical reading scheme, fingerprint data is read mainly using reflection of light and an image sensor (CCD) and collated (e.g., Seigo Igaki et al., Japanese Patent Laid-Open No. 61-221883). A scheme of reading a pressure difference by the three-dimensional pattern of the skin surface of a finger using a piezoelectric thin film has also been developed (e.g., Masanori Sumihara et al., Japanese Patent Laid-Open No. 5-61965).
An authentication scheme of replacing a change in electric characteristic due to contact of a skin with an electrical signal distribution by detecting a resistive or capacitive change amount using a pressure sensitive sheet so as to detect a fingerprint has also been proposed (e.g., Kazuhiro Itsumi et al., Japanese Patent Laid-Open No. 7-168930).
In the above prior arts, however, the optical reading scheme is difficult to make a compact and versatile system, and its application purpose is limited. The scheme of detecting the three-dimensional pattern of the skin surface of a finger using a pressure sensitive sheet or the like is difficult to put into practical use or is unreliable because a special material is required and fabrication is difficult.
“Marco Tartagni” et al. have developed a capacitive fingerprint sensor using an LSI manufacturing technology (Marco Tartagni and Robert Guerrieri, A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme, 1997 IEEE International Solid-State Circuits Conference, pp. 200-201 (1997)).
In this fingerprint sensor, the three-dimensional pattern of a skin is detected using a feedback static capacitance scheme by a sensor chip in which small capacitive detection sensors are two-dimensionally arrayed.
In the capacitive detection sensor, two plates are formed on the uppermost layer of an LSI, and a passivation film is formed on the plates. In this capacitive detection sensor, a skin surface functioning as a third plate is isolated by an insulating layer formed from air, and sensing is performed using the difference in distance, thereby detecting a fingerprint. As characteristic features of a fingerprint authentication system using this structure, no special interface is necessary, and a compact system can be constructed, unlike the conventional optical scheme.
In principle, a fingerprint sensor using a capacitive detection sensor is obtained by forming a lower electrode on a semiconductor substrate and forming a passivation film on the resultant structure. A capacitance by the skin and lower electrode is detected through the passivation film, thereby detecting the fine three-dimensional pattern of the skin surface of a finger. As shown in FIG. 21, a capacitive detection sensor is formed from a lower electrode 2102 formed on a semiconductor substrate 2101 via an interlevel dielectric 2101a, and a passivation film 2103 that covers the resultant structure.
A fingerprint sensor chip is formed by arraying a plurality of capacitive detection sensors on the semiconductor substrate 2101 in a matrix. Although not illustrated in FIG. 21, an integrated circuit having, e.g., a plurality of MOS transistors and interconnection structure is formed on the semiconductor substrate 2101 under the interlevel dielectric 2101a. The lower electrodes 2102 are connected to the integrated circuit by interconnections (not shown). A capacitance generated in the plurality of lower electrodes 2102 is detected by a detection circuit formed on the integrated circuit and output.
In this sensor chip, when a finger whose fingerprint is to be detected comes into contact with the passivation film 2103, the skin in contact with the passivation film 2103 functions as an electrode on each lower electrode 2102, and a capacitance is formed between the skin surface and the lower electrode 2102. The formed capacitance is detected by the detection circuit through the interconnection (not shown) connected to the lower electrode 2102.
A fingerprint is formed by the three-dimensional pattern of a skin. Hence, the distance between each lower electrode 2102 and a skin serving as an electrode in contact with the passivation film 2103 changes between the projection and recess of the fingerprint. The difference in distance is detected as a difference in capacitance. When the distribution of different capacitances from the respective lower electrode 2102 is detected, a fingerprint pattern can be obtained. As described above, a sensor chip using capacitive detection sensors serves as a surface shape recognition sensor capable of sensing a fine three-dimensional pattern of a skin.
In the above-described sensor chip using capacitive detection sensors, however, since a skin serves as an electrode, static electricity generated at the fingertip readily causes electrostatic destruction in an integrated circuit such as a sensor circuit incorporated in the sensor chip.
To prevent the above-described electrostatic destruction of an electrostatic capacitance fingerprint sensor, a surface shape recognition sensor having an electrostatic capacitive detection sensor having a sectional structure as shown in FIG. 22 has been proposed. The sensor shown in FIG. 22 will be described. The sensor has a lower electrode 2203 formed on a semiconductor substrate 2201 via an interlevel dielectric 2202, a plate-shaped deformable upper electrode 2204 which is separated from the lower electrode 2203 at a predetermined interval, and a support electrode 2205 laid out around the lower electrode 2203 to support the upper electrode 2204 while being insulated and isolated from the lower electrode 2203.
In the sensor having the above arrangement, when a finger to be subjected to fingerprint detection comes into contact with the upper electrode 2204, the pressure from the finger deflects the upper electrode 2204 toward the lower electrode 2203 to change the electrostatic capacitance formed between the lower electrode 2203 and the upper electrode 2204. This change in electrostatic capacitance is detected by a detection circuit (not shown) on the semiconductor substrate 2201 through an interconnection (not shown) connected to the lower electrode 2203. In this surface shape recognition sensor, when the upper electrode 2204 is grounded through the conductive support electrode 2205, static electricity generated at the fingertip and discharged to the upper electrode 2204 flows to ground through the support electrode 2205. For this reason, the detection circuit incorporated under the lower electrode 2203 is protected from electrostatic destruction.
However, the above-described conventional fingerprint sensors can obtain no desired high sensitivity. For example, in the fingerprint sensor having the structure shown in FIG. 21, since the sensitivity largely changes depending on the state of a finger surface, it is difficult to obtain a high sensitivity. In the fingerprint sensor having the structure shown in FIG. 22, since no large change in upper electrode is obtained, no high sensitivity can be obtained.